The manner of action of the motor cells (Fig. 5, c, e) has not been clearly made out. By different investigators the curvature has been successively attributed to an accelerated growth of the convex (upper) side of the tendril, a retarded growth of the concave (lower) side, an increased turgor in the region of curvature with an increased extensibility of the walls of the convex side, and an increased turgor of the convex side only, coupled with an increased extensibility of the walls of the cells on this side. But recently the hypothesis that the curvature is due to the increased turgor of the concave side, with an increased extensibility of the walls of this side in one direction only, has been offered. Whether the last offers the true solution of the problem or not remains to be proved. However, it accounts for certain features of the curvature irreconcilable with previous explanations. The conditions of the motor cells after curvature may be seen in Fig. 5.
In plants provided with cushions of tissue—pulvini—for the purpose of rapidly displacing leaf stalks and other organs, the movement is effected by a direct contraction of the cells on the side toward which the organ is curved. It will be quite remarkable if it should be found that the rapid movements of the "sensitive plants" are effected by the contraction of the motor cells, and the slower movements of tendrils and other organs by the expansion of these cells.
Of the ultimate molecular changes ensuing in the cells of the motor or perceptive zones, as well as in the transmitting tissues, nothing is known, except that in such cells the metabolic action is necessarily very rapid. In this the physiologist confronts a problem which may not reach its final solution until the ultimate organization of protoplasm is at least approximately ascertained.
The delicacy of the mechanism of irritability in certain instances is such that the amount and intensity of the stimulus necessary to secure a reaction are extremely small. It has been found that tendrils would respond to the contact of a weight not greater than one five-thousandth of a milligramme, and that a plant in a dark chamber would curve toward the light afforded by a single spark from a small condenser coil, or to a light too diffuse to cast a shadow perceptible to the human eye. Some of the lower free-swimming forms have been found susceptible to the presence of an amount of oxygen so small as the one-trillionth part of a gramme, which is very nearly the atomic limit of this substance. This delicacy of perception is doubtless beyond that of the senses of the higher animals.
At all times the amount of a stimulus necessary to produce a response increases with the amount previously acting upon the plant. Thus a plant in total darkness will react to an extremely small amount of light, as has been pointed out; but, if placed in
